The present invention relates to a PSK (phase-shift-keying) transmitter used for a CATV (community antenna television) system or the like, and more particularly to a PSK transmitter in which a sufficient DU ratio (desired-to-undesired signal ratio) can be obtained.
Conventionally known is a PSK transmitter in which a PSK modulation system is employed for transmitting various information. FIG. 1 shows the arrangement of the conventional PSK transmitter for PSK communications. In FIG. 1, reference numeral 1 designates an oscillator circuit having a PSK carrier frequency, 2 designates an amplifier for amplifying the carrier generated by the oscillator circuit 1, and 3 designates a PSK-modulated data signal to be transmitted. Reference numeral 4 designates a switching signal for ON/OFF controlling a switching circuit 8 described below, 5 designates a control circuit constituted by a CPU (central processor unit) for generating the data 3 and the switching signal 4, and 6 designates a PSK modulator for PSK-modulating the carrier generated by the oscillator circuit 1 and amplified by the amplifier 2, with the data 3 generated by the control circuit 5. Reference numeral 7 designates an amplifier for amplifying the PSK-modulated signal which has been PSK-modulated by the PSK modulator 6, 8 designates a switching circuit which is turned on/off in accordance with the state of the switching signal 4 for permitting/inhibiting passage of the PSK-modulated signal from the PSK modulator 6 amplified by the amplifier 7, 9 designates a bandpass filter for passing only signals having a frequency approximate to that of the PSK-modulated signal passed through the bandpass filter 9.
In the thus-arranged PSK transmitter, a carrier of a frequency f.sub.c generated by the oscillator circuit 1 is amplified by the amplifier 2 to a required level, and then is applied to the PSK modulator 6. The PSK modulator 6, employing a two-phase PSK modulation system, modulates the carrier f.sub.c with the data 3 generated by the control circuit 5. As a result, the carrier is modulated so as to be inverted in phase by 180 degrees at the leading and trailing edges of the data 3 as shown in FIG. 2, and the modulated signal is outputted as a PSK-modulated signal. This PSK modulated signal is further amplified by the amplifier 7 to a required level and is applied to the switching circuit 8 controlled in accordance with the switching signal 4 produced by the control circuit 5. As shown in FIG. 3, the switching circuit 8 is turned off to cut off the PSK-modulated signal applied thereto when the switching signal 4 has a level V.sub.L (0) and is turned on to pass the PSK-modulated signal for periods(T.sub.1 -T.sub.2 and T.sub.3 -T.sub.4) when the switching signal 4 has a level V.sub.H (1), thereby to control the transmission of the PSK-modulated signal. The output of the switching circuit 8 is subjected to attenuation ATT1 when in the on-state for minimal relative loss so as to have substantially the same level as an input signal received by the switching circuit 8. The output signal of the switching circuit 8 which has been subjected to the off-state-attenuation ATT2 or the on-state-attenuation ATT1 is limited in frequency band by the bandpass filter 9 and is applied to the output terminal 10.
In such a PSK transmitter, it is necessary to set a differential DU (desired to undesired signal) ratio between the on-state-attenuation ATT1 and the off-state-attenuation ATT2 so as to prevent signal reception from being carried out in the PSK receiver when the switching circuit 8 is in the off state. The DU ratio is obtained by the following inequality: EQU D/U&gt;V.sub.i +10 log n-V.sub.min (dB)
where V.sub.min represents the minimum reception sensitivity (dB.mu.) of the PSK receiver, V.sub.i represents the maximum level (dB.mu.) of the carrier received by the PSK receiver from the PSK transmitter, and n represents the number of PSK transmitters.
FIG. 4 shows the relationship between the DU ratio and the number of PSK transmitters, with the minimum reception sensitivity V.sub.min of a PSK receiver fixed because the minimum reception sensitivity V.sub.min is generally considered to be about 50 dB.mu.. The DU ratio and the number of PSK transmitters are represented by the ordinate and the abscissa, respectively. As seen from FIG. 4, the larger the maximum carrier reception level V.sub.i in the PSK receiver, the larger the required DU ratio, and the larger the number n of PSK transmitters n, the larger the required DU ratio.
An example of a PSK communication system using a plurality of PSK transmitters and a single PSK receiver as described above is a CATV system using a PSK receiver provided at the head end (center) and PSK transmitters respectively provided at the various customer terminals. In a CATV system having several tens of thousands of terminals, the terminals are arranged in groups of hundreds of terminals or thousands of terminals, and a switching circuit is provided for every group. In a CATV system having a relatively small number of terminals, however, the terminals are not grouped, and therefore the switching circuit 8 of the PSK transmitter is required to maintain a DU ratio as shown in FIG. 4.
It is generally considered that the minimum reception sensitivity V.sub.min and the maximum carrier reception level V.sub.i of a PSK receiver are set to about 50 dB.mu. and 100 dB.mu., with the number of PSK transmitters being about 3000. In this case, the DU ratio in the switching circuit 8 is required to be no less than 84.77 dB. However, it is difficult to easily and inexpensively realize a switching circuit having such a property, which is a heretofore unresolved problem in the conventional PSK transmitter.